In the conventional plastic injection molding that is conducted while controlling the entire metallic mold at a low temperature and moreover to a certain temperature, the material resin of molten thermoplastic resin, etc. is speedily cooled down from the surface in contact with the metallic mold and a thick solidified layer is formed, or the transcription between the metallic mold surface and the product surface can not be done sufficiently attributable to the heat shrinkage, and the transcription defects such as flow marks, weld marks, shrinkage, etc. are caused. This is because the hard solidified layer of resin surface hinders the deformation due to the internal pressure of the resin, the resin surface closely adheres to the metallic mold surface and prevents the transcription.
Further, there is such a case of including such filler as glass fiber and beads, and rubber particle as butadiene, etc. into the resin for reinforcing the mechanical strength of the resin, and if these should be contained in the resin, such filler as glass, etc and rubber particle as butadiene, etc. are left over on the resin surface by its thermal shrinkage difference, forming precise irregularities, worsening the transcription properties and damaging the outside appearance of product.
This kind of transcription defect of such product not only damages the outside appearance of product to decrease the product value but also hinders the uniform coating to damage the aesthetic appearance in case of providing the coating on the product surface, and there is such a problem as the cost for repair becomes higher. Further, in an extreme case, the surface smoothness accuracy or the outside appearance being requested as to such product can not be satisfied, thus making the product to be value-less.
This kind of transcription defect can be improved to a certain degree by making the resin pressure inside the metallic mold higher, pressing the resin against the metallic mold surface, by reducing the volume of filler, rubber grain, etc. being contained into the material resin, by making the diameter of grain size smaller, or by improving the material resin itself. However, making the metallic mold internal pressure higher necessitates the use of metallic a mold of higher strength and a large type molding machine capable of generating a high pressure, and causes such new problems as the higher cost, the internal strain of product itself and the generation of deformations. Further, the improvement of the resin itself causes such a problem that the performance of necessary strength and the like can not be satisfied because the serious consideration is taken to the outside appearance.
The highest effect against the transcription defect is to heat the face of metallic mold in contact with the product to the level higher than the glass transfer temperature if the material resin is non-crystal and to heat the said face to the level higher than the fusion temperature if the resin is crystal. This is because the solidified layer of resin in contact with the metallic mold tends to become thinner and deformed and to transcript the surface of metallic mold faithfully by making the surface of the metallic mold higher in temperature.
However, if the surface of metallic mold should simply be heated, the product surface is exposed to a high temperature for a longer period than necessary, there appear such new problems as the appearance of cumber and deformation to the product, and there also appears such an harmful influence that the molding cycle is elongated and the product cost becomes higher. Also, if the necessary high metallic mold surface temperature should not be reached or if the metallic mold surface temperature should be too high, the outside appearance needed to the product and the shape accuracy are damaged, thus causing the loss in production. Therefore, it is essential not only to detect the temperature of the metallic mold surface with high accuracy but also to improve the temperature of the metallic mold, and control the heating and cooling temperatures of metallic mold surface to the targeted temperature and time with high accuracy within a short time based on the detected temperature.
Conventionally, the method for controlling the metallic mold temperature is not to directly detect the surface temperature of metallic mold, but to detect the temperature of heating thermal medium and cooling thermal medium being supplied into the metallic mold using the temperature detection sensor inserted into the medium piping surface or into the piping, thus controlling the medium temperature based on this detection temperature and controlling the metallic mold temperature. Further, as the method for directly detecting the metallic mold surface temperature, such a method is being used as installing the temperature detection sensor inside the metallic mold insert die by the machining, and fitting it into the surface of metallic mold together with this insert die.
Moreover, as the units attempting to improve the responsibility of metallic mold temperature control, to shorten the metallic mold heating time and to raise the control accuracy of metallic mold temperature, a variety of the following units are being proposed.
(1) The unit installing an induction coil around the outer circumference of metallic mold (Gazette of Japan Unexamined Utility Model Publication No. S62-111832). PA0 (2) The unit installing a high frequency induction coil consisting of the copper pipe to the metallic mold to flow the cooling water inside the pipe (Gazette of Japanese Unexamined Patent Publication No. S63-15707). PA0 (3) The unit where the mobile insert die provided with an electric heater can be inserted into or pulled out of the metallic mold (Gazette of Japanese Unexamined Patent Publication No. S63-15719). PA0 (4) The unit where such a device is installed for changing over the speedily heating or cooling of the medium for metallic mold temperature control or the heated or cooled thermal medium and supplying it to the outside of metallic mold, and the heating or cooling thermal medium being supplied can heat or cool the metallic mold through the temperature adjusting medium circulation channel inside the metallic mold (Gazette of Japanese Unexamined Patent Publication No. S62-15707, Gazette of Japanese Unexamined Patent Publication No. S62-208918, Gazette of Japanese Examined Patent Publication No. H7-25115).
As described earlier, the transcription defect of the product can be improved by heating to the high temperature the surface where the material resin of metallic mold flows, but if a long time should be taken to the heating and cooling, then there may appear such new problems that the molding cycle is elongated, the productivity falls down, and the rise of product cost will be invited and the product itself will be cumbered. Further, if the control accuracy of metallic mold surface temperature should be worse, and the molding should be done at the temperature being set as the surface temperature of metallic mold, namely the glass transition temperature in case that the material resin is non-crystal, or at the metallic mold surface temperature lower than the fusion temperature in case that the resin is crystal, the transcription defect is caused to the product generating the production loss, and to the contrary, if the metallic mold surface temperature should become too high or the product has been exposed to a high temperature for many hours, the shrinkage volume of material resin becomes further greater, such rubber grains as the filler, butadiene, etc. like the glass and so forth are left over on the resin surface by the post-shrinkage, etc, and there is also a problem that the transcription defect occurs again.
Therefore, for obtaining the high transcription product, it is necessary not only to detect the surface temperature of the metallic mold with high accuracy but also to improve the temperature responsiveness of the metallic mold, and to control the heating and cooling temperatures of the metallic mold surface within a short time, with high accuracy to the targeted temperature and time on the based of the detected temperature.
However, also with the method for controlling the surface temperature of the metallic mold, in the method for detecting the temperatures of heating thermal medium and cooling thermal medium to be supplied to the metallic mold without detecting directly the surface temperature of the metallic mold like the conventional method and for controlling the thermal medium temperature based on this detected temperature to control the surface temperature of the metallic mold, there appears a difference to the temperature and the heating and the cooling time between the heating and cooling medium temperatures and the surface temperature of the metallic mold, and that difference is changed by the controlled temperature and doesn't remain constant. Therefore, it is difficult to previously predict all the temperature and time difference between these heating and cooling thermal medium temperatures and the surface temperature of metallic mold and to heat- or cool-control the surface temperature of metallic mold with high accuracy to the targeted temperature and time.
In the method for installing the temperature detection sensor into the metallic mold insert die by its machining, fitting whole the insert die into the surface of the metallic mold to detect the surface temperature of metallic mold and controlling the heating and cooling thermal medium temperatures based on this detected temperature to control the surface temperature of the metallic mold, there is the special feature capable of decreasing the timely difference of temperature and temperature change being caused between the heating and cooling thermal medium temperatures and the surface temperature of metallic mold.
However, in this system, because the temperature detection sensor is installed into the insert die and the metallic mold by its machining, there exists the gap between the temperature detection sensor and the insert die and the metallic mold, the heat transfer is worse, and because no means for controlling the temperature of the metallic mold is not installed to the insert die itself to which this kind of temperature detection sensor has been installed, like the channel passing through the heating and cooling thermal medium, etc., there appears a timely difference to the temperature and temperature change between the surface temperature of surrounding metallic mold and the detected temperature by the temperature detection sensor.
In addition to the said descriptions, because in the conventional metallic mold structure the metallic mold temperature adjusting thermal medium tube channel and the temperature detection sensor of metallic mold separates by more than several tens of millimeters from the surface of metallic mold, there appears the temperature and timely difference from this distance problem, and becomes a hindrance in controlling the surface temperature of metallic mold with high accuracy.
Further, also concerning the method attempting to improve the temperature responsibility of the metallic mold, to shorten the heating time of the metallic mold and to enhance the control accuracy of the metallic mold temperature, there are some problems as follows to the conventional method.
The above mentioned unit of Items (1) and (2) installing the induction coil has many restrictions for its incorporation into the metallic mold, and the metallic mold is enlarged. The unit in Item (3) that has installed a mobile insert die having an electric heater must provide the insertion area of mobile insert die on the metallic mold, and the structure of metallic mold becomes complex. In addition, this unit has no universality to the heating means. Further, because the natural cooling is adopted, the molding cycle becomes longer. The unit in Item (4) for heating and cooling the surface of metallic mold by passing the heating and cooling thermal media through the temperature adjusting thermal medium circulation pipe channel from the outside unit has the universality, and there is also such a merit that the structure of the metallic mold requires no special machining. However, if the temperature adjusting thermal medium circulation channel inside the metallic mold in the range which is usually installed should be used, the thermal capacity of heating or cooling metallic mold portion is great and hence there appears the loss of heat, and the time is required for heating and cooling. Further, to the end there is such a problem that the control accuracy of temperature becomes worse because the response of surface temperature of metallic mold is late.